Apparatus for correcting perspective distortions are known as PC-objectives or shift objectives when the imaging optical system is configured as an objective. These apparatus compensate for perspective distortions such as tumbling lines. Objectives of this kind are known and can be displaced vertically, horizontally and diagonally on the camera and are used to correct perspectives for pictures primarily in architectural and industrial photography.
U.S. Pat. No. 2,931,268 discloses an apparatus wherein two plates are mounted in respective dove-tail guides wherein they can be displaced relative to the film plane with spindles selectively in the x or y direction. This type of plate movement requires a substantial amount of space and is very expensive since many parts having very tight tolerances relative to each other are required. This applies in the same manner to objectives having a coupling device disclosed in published British patent application No. 2,012,974.
U.S. Pat. No. 4,081,812 discloses a photographic objective wherein the dove-tail guide is avoided.
In the photographic lens of U.S. Pat. No. 4,081,812, rotational rings are provided between an adjusting frame of the objective and a swing-angle transmission system for movement in the x and y directions with these rotational rings being disposed one over the other. The center ring is fixedly mounted so as to be centered to the camera and the radial and axial guide is defined by two further rings. These two further rings transmit with their rotation the movement to the adjusting frame of the objective via control curves and connecting parts. The high space requirement and the problem of the very tight tolerances here too do not provide an optimal solution. The same applies for the so-called shift adapter disclosed in German Patent No. 3,436,886 wherein the shift adapter is disclosed as being mounted between the objective and the camera.
U.S. Pat. No. 3,713,725 discloses a PC-objective wherein the adjustment perpendicular to the optical axis takes place by means of two interconnected eccentric rings. The space requirement is less than the other solutions discussed above but the tolerance problem is shifted from many linear surfaces to rounded surfaces without a substantial reduction of the surfaces which have to be precisely machined. Furthermore, only a very small adjusting range can be realized with this PC-objective.